[Arran]

No, this is the product of a defective local oscillator, most likely a bad resistor in the circuit, but the old paper caps are likely not helping, most of those can be melted out and restuffed for looks however. As for disagreeing with the design by putting a plexiglass bottom under the chassis, there may have been a reason why the set had a metal bottom other then preventing stray fingers from getting a shock, sometimes the bottom covers acted as a shield against electrical interference, sometimes they also acted as protection in case a component shorted out and melted down, which did happen sometimes.
Regards
Arran